1. Field
Embodiments relate to a system for separating a diced semiconductor die from an adhesive tape.
2. Description of the Related Art
The strong growth in demand for portable consumer electronics is driving the need for high-capacity storage devices. Non-volatile semiconductor memory devices, such as flash memory storage cards, are becoming widely used to meet the ever-growing demands on digital information storage and exchange. Their portability, versatility and rugged design, along with their high reliability and large capacity, have made such memory devices ideal for use in a wide variety of electronic devices, including for example digital cameras, digital music players, video game consoles, PDAs and cellular telephones.
While a wide variety of packaging configurations are known, flash memory storage cards may in general be fabricated as system-in-a-package (SiP) or multichip modules (MCM), where a plurality of die are mounted on a substrate in a so-called three-dimensional stacked configuration. An edge view of a conventional semiconductor package 20 (without molding compound) is shown in prior art FIG. 1. Typical packages include a plurality of semiconductor die 22, 24 mounted to a substrate 26. Although not shown in FIG. 1, the semiconductor die are formed with wire bond pads on an upper surface of the die. Substrate 26 may be formed of an electrically insulating core sandwiched between upper and lower conductive layers. The upper and/or lower conductive layers may be etched to form conductance patterns including electrical leads and contact pads. Wire bonds 30 are connected between the wire bond pads of the semiconductor die 22, 24 and the electrical leads of the substrate 26 to electrically couple the semiconductor die to the substrate. The contact pads on the substrate in turn provide an electrical path between the die and a host device. Once electrical connections between the die and substrate are made, the assembly is then typically encased in a molding compound to provide a protective package.
In order to form a semiconductor package, a die bonding process is performed where a semiconductor die is diced from a wafer, picked up from an adhesive tape and bonded to a substrate. Prior art FIG. 2 shows a wafer 40 including a plurality of semiconductor die, for example die 22 (only some of which are numbered in FIG. 2). Each semiconductor die 22 on wafer 40 has been processed to include an integrated circuit as is known in the art capable of performing a specified electronic function. After testing the die 22 for bad die, the wafer may be placed on an adhesive film 44, referred to as a die attach film (DAF) tape, and then diced for example by saw or laser. The DAF tape may be formed of a die attach film adhered to a tape, and upon separation of the die from the tape, the film may remain affixed to a bottom surface of the die. The dicing process separates the wafer into individual semiconductor die 22, which remain affixed to the DAF tape. FIG. 2 shows wafer 40 affixed to a DAF tape 44.
In order to detach the individual die, the wafer and DAF tape are situated in a process tool, portions of which are shown in prior art FIG. 3. FIG. 3 shows a die ejector tool 50 including a vacuum chuck 52 on which is supported the wafer 40 and DAF tape 44. In order to lift the die off of the DAF tape 44, a pick-up tool 60 is provided including a vacuum tip 62. The pick-up tool 60 comes down over a die 22 to be removed from the DAF tape 44, the vacuum is applied to the tip 62, and the die 22 is pulled up off the tape 44. The pick-up tool then transports the die 22 for attachment to the substrate or for transport elsewhere.
FIG. 4 shows a bottom view of a conventional vacuum tip 62. The tip includes a plurality of vacuum holes 64, some of which are numbered in FIG. 4, for communicating the negative pressure to the surface of the vacuum tip 62. The negative pressure at the vacuum holes 64 hold the die 22 on the pick-up tool 60. In conventional vacuum tips, the vacuum holes 64 are uniformly distributed across the surface of the tip, for example as shown in FIG. 4.
One drawback to conventional vacuum tips is that, while the vacuum holes 64 exert a uniform pressure across the surface of a die to be lifted off of a DAF tape, the die is not peeled off from the DAF tape with a uniform pressure. In particular, during dicing of the wafer with a blade, the blade causes shear and normal forces near the edges of the die where it is cut. The shear and normal forces increase the adhesive force between the die and DAF tape near the edges of the die. These adhesive forces are a function of the distance, x, from the edges of a die, and decrease with a square of the distance away from the edge of a die. In one example, the adhesive force F(x) exerted by the DAF tape to the die is proportional to some constant, K, divided by a square of the distance, x, from the edge of the die:F(x):K/(1+x2).The constant, K, is the sum of different constants arising from the different mechanisms of adhesion. For example, chemical bonds occur between the adhesive and the dicing tape, which chemical bonds may be strengthened upon heating due to being cut with a blade or laser. Additionally, electrostatic forces may also result near the die edges as a result of the dicing. Further, van der Waals forces develop within the molecules of the DAF and the dicing tape.
Prior art FIG. 5 shows the upward forces Fu on die 22 resulting from the vacuum tip 62, and the downward forces Fd on die 22 resulting from the adhesion between the die 22 and DAF tape 44. As noted above, where the upward force of the vacuum tip is uniform, the downward force varies by a square of the distance away from the edge. Prior art FIG. 5 shows the net resultant forces Fn on the die as a result of the upward forces Fu and the downward forces Fd. As seen in FIG. 6, the net forces on die 22 are greater toward a middle of the die than at the edges.
The net result of this uneven force profile is that the die 22 may bend when being picked up by the vacuum tip 62 from the DAF tape in conventional die ejector tools. This scenario is shown in prior art FIG. 7. The bending of the die can have several harmful effects. The die may not be secure on the vacuum tip 62, and may fall off the tip 62. Moreover, the die may crack when it is bent, and/or the conductive traces formed on the surface of the semiconductor die 22 may get damaged when the die is bent or shift so as to electrically short against an adjacent conductive trace. This is especially true today, where the thicknesses of semiconductor die have been reduced in some examples to 40 or 50 microns (μm).
Even where no such damage occurs to the die, a further problem may occur when the bent die is mounted on a substrate, such as substrate 70 shown in prior art FIG. 8. Given the concave shape of the die 22 against the surface of the substrate 70, air bubbles 72 may form between the die and substrate. These air bubbles may cause the die to delaminate from the substrate, for example when the die and substrate are heated in the encapsulation process, due to the expansion of the air bubbles.